Electrically powered vehicles are mounted with storage batteries such as lead-acid batteries, nickel metal hydride batteries, and lithium-ion batteries. The power needed by hybrid vehicles or electric cars for traveling is provided by these batteries.
A storage battery mounted on the vehicle is used in a wide temperature range and has maximum permissible power commensurate with the temperature of the storage battery. If the storage battery is charged or discharged in excess of the maximum permissible power, it could be overcharged or overdischarged.
A storage battery generally has small maximum permissible power at a low temperature and large maximum permissible power at a high temperature. In spite of the large maximum permissible power at the high temperature, the use of the storage battery in the high-temperature state promotes its deterioration. Meanwhile, as the state of charge (SOC) of a storage battery becomes higher, its maximum permissible charging power becomes smaller, and its maximum permissible discharging power becomes larger; as the SOC of the storage battery becomes lower, its maximum permissible discharging power becomes lower, and its maximum permissible charging power becomes larger. To use the storage battery safely requires performing its charging and discharging control within a range of its maximum permissible power.
Patent Literature 1 cited below discloses a technique of calculating the permissible discharging power suitable for each single cell in order to perform optimum charging and discharging control even when the single cells making up a battery pack are subject to temperature variation.